1. Field of Invention
The present invention relates to a method for forming a metallic layer. More particularly, the present invention relates to a method of forming a metallic layer capable of preventing a masking effect during subsequent etching operation.
2. Description of Related Art
Aluminum is a commonly used conductive material in the fabrication of VLSI circuits. Besides having a low material cost and a good electrical conductivity, aluminum is easy to deposit and etch, and has good adhesive strength with silicon as well. Therefore, aluminum is often used as a runner for semiconductor devices. However, as the level of integration for semiconductor devices is increased, the use of aluminum as a runner for semiconductor devices poses some problems. First, when the line-width of an aluminum wire is narrowed because of device miniaturization, electrormigration will occur and will ultimately lead to an open aluminum wire. Secondly, since aluminum has a definite solid solubility in silicon, aluminum and silicon can inter-diffuse quite easily when heated to a moderately high temperature. Hence, spikes can easily form at their junction interface. Consequently, it is normal to employ an alloy, which is a blend of copper and aluminum, as the material for forming the runners in order to reduce electromigration and spiking problems.
In general, aluminum-copper alloy is used in conventional fabrication processes because extracts of copper inside the alloy are able to lower the rate of electromigration in aluminum. However, when there are too many copper extracts or the size of each individual copper extract is too big, undesirable effects such as the masking effect can easily occur. Normally, when the alloy is heated to a temperature above 120.degree. C., the quantity of copper extracts will start to increase while the size of each extract will grow. The situation is particularly critical when the metallic layer needs to be reworked after finding some defects in an after develop inspection (ADI). In rework processing, the photoresist layer needs to be removed and the surface thoroughly cleaned using plasma. All the while, the copper extracts within the metallic alloy will also grow larger, thus intensifying the masking effect. One method of solving the problem is to use a chemical formula having a lower selectivity ratio to etch the wafer and then using a highly energetic beam of ions to bombard and remove the copper extracts. However, as the current trend is towards a more precise fabricating process, which means a higher etching selectivity ratio and a thinner photoresist layer, the above method has definite limitations.
FIG. 1 is a block diagram showing the conventional manufacturing flow for forming a metallic layer. In FIG. 1, label 10 represents the step of forming a layer of aluminum-copper alloy, label 12 represents the step of forming a layer of photoresist, label 14 represents the step of developing the photoresist layer, label 16 represents the step of performing an after development inspection, label 18 represents the step of performing subsequent processing operations and label 13 represent the step of removing the photoresist layer. Fabrication of the metallic layer includes first forming a metallic layer 10 over a semiconductor substrate having a MOS devices already formed thereon, where the metallic layer can be aluminum-copper alloy functioning as multilevel interconnect. Before the formation of a photoresist layer 12, an anti-reflection layer can be deposited over the metallic layer 10. The photoresist layer 12 is formed over the metallic layer 10 and then the photoresist layer 12 is developed 14. Subsequently, an after development inspection 16 is carried out to ensure a proper photolithographic operation has been performed. If the outcome of the inspection 16 is satisfactory, subsequent photolithographic and etching operations 18 are carried out. On the other hand, if the quality after inspection 16 is unsatisfactory, the photoresist layer will be removed 13 the surface of the wafer will be cleaned using plasma. The whole procedure of photoresist layer formation 12, photoresist development 14 and inspection 16 will be repeated. However, as the temperature increases to between 120.degree. C. to 350.degree. C., a large quantity of copper extracts will emerge and each extract will grow in size, too. The increase in copper extracts is considerable especially when rework is needed and the photoresist layer must be removed 13 due to a poor development 14. In a subsequent processing step 18 as shown in FIG. 1, when the aluminum-copper alloy is dry-ctchcd, the chlorine plasma used in the etching operation can react with the copper extracts in the alloy to form copper chloride compound. Since copper chloride is non-volatile, its removal is very difficult and can easily lead to a masking effect.
In light of the foregoing, there is a need to improve the method of forming metallic layer on a semiconductor wafer.